This invention relates to optical discs, and more particularly to an improved method and apparatus for making double-sided, 3-layer and 4-layer optical discs, such as DVD-14 and DVD-18 discs.
Very high capacity optical discs have two information layers that can be read with the same laser beam. The read head focuses the beam on a selected one of the two information layers. Each information layer is a reflective coating that modulates and reflects the laser beam. The information layer closest to the side of the disc where the read head is positioned has a semireflective coating. When the laser beam is focused on this layer, sufficient light is reflected to allow the data on this layer to be read. The information layer that is remote from the side of the disc where the read head is positioned has a fully reflective coating. When the laser beam is focused on this layer, light that passes through the semireflective layer is modulated by the fully reflective layer and passes once again through the semireflective layer.
Single-layer DVD discs are made using the same type of equipment that has been used for many years in the manufacture of CDs. A substrate is injection molded to have an information layer in the form of pits and lands. The information layer is then sputtered with a fully reflective aluminum coating, and a protective coating may be applied on top of the sputtered surface. Because a DVD disc made this way has only half the thickness of a CD yet the finished DVD must be as thick as a CD, two DVD discs are bonded together to make a finished DVD. If the DVD is to have only a single information layer, then the second disc may be a blank. If the DVD is to have two information layers, two single-layer discs may be bonded together. (If reading is to take place from only one side, one of the reflective layers must be semireflective; if reading is to take place from both sides, then both reflective layers should be fully reflective.) But as long as no more than two information layers are required for a DVD, standard CD production techniques can be used to make each disc.
The problem is that standard CD production techniques are not adequate or manufacturing dual-layer optical discs that have three or more reflective coatings. The substrate and one semireflective coating can be made in the usual way--injection molding followed by sputtering (using gold to obtain a semireflective coating rather than aluminum as in the case of a CD where a fully reflective coating is needed). But the injection molding equipment used in CD production facilities can not be used to mold the additional pit-and-land structure required for a second information layer. Therefore, what has been proposed in the prior art is to coat the semireflective coating with a viscous resin and to stamp the second information layer in it (followed by conventional sputtering and application of a conformal coating). However, the companies presently in the CD production business do not generally have stamping equipment that is adequate for this purpose, and therefore new assembly lines have been proposed for producing dual-layer DVD discs.
Before proceeding, it is necessary to define certain terms as they are used herein. Because a CD has a single substrate structure that is 1.2-mm thick and a DVD has two 0.6-mm thick substrate structures bonded together, the word "disc" herein has different meanings depending on the context. It can refer to the 0.6-mm substrate structure that makes up half of a DVD, or it can refer to the entire 1.2-mm thick DVD itself. Also, the word substrate refers to an injection-molded plastic disc, while the term substrate structure refers to the same disc together with whatever coatings (sputtered, etc.) may be on it.
It is also important to understand what is meant herein by the terms stamping and stamper. Injection molding is a technique in which liquid plastic is injected into an enclosed volume (a mold). Stamping is a technique in which a press is caused to move against a solid sheet or a viscous layer of material to impress a pattern in it, the operation usually not taking place in an enclosed space. In the days of vinyl records, which records were made by stamping machines, the pattern in the press that was impressed into a flattened vinyl "biscuit" was naturally called a stamper. Since injection molds also require a pattern to impress into the injected plastic, and the pattern for a CD is made in a way reminiscent of the way vinyl record stampers were made, it was natural for the CD industry to call the pattern placed in a mold a "stamper" even though it is not used in a stamping machine. Thus as used herein the operation called stamping involves a press which moves, usually in open space, to embosses a pattern into a viscous material, and the word stamper refers to such a pattern whether it is used in stamping equipment or injection molding equipment.
The copending application of William R. Mueller, entitled "Dual-Layer DVD Disc, and Method and Apparatus for making Same," filed on even day herewith and hereby incorporated by reference, discloses a method (and associated manufacturing apparatus and finished product) for making dual-layer 0.6-mm DVD discs using injection molding equipment that does not also require the use of stamping equipment. This allows dual-layer 0.6-mm DVD discs to be made using primarily the equipment presently found on the assembly lines of CD and DVD manufacturers. The Mueller method entails making two substrates, each with a metallic information layer, in the usual way--with injection molding equipment. The first substrate has a semireflective (gold) information layer. The second substrate has a fully reflective (aluminum) information layer. The semireflective information layer on the first substrate is coated with a very thin layer of glue, and the fully reflective information layer on the second substrate is then transferred to the layer of glue. In this way, a 0.6-mm dual-layer disc is formed without requiring additional stamping equipment, and with both metallic layers having been sputtered onto molded substrates.
Because it is desired that the fully reflective metallic layer release from the second substrate during the transfer step, the second substrate--which does not end up as part of the finished DVD--can be made of polymethyl methacrylate (PMMA) rather than the polycarbonate of the first substrate. The fact that PMMA does not bond well to aluminum, and adhesion is only through molecular roughness, is advantageous.
The usual protective coating may then be applied as the last step in the making of the dual-layer 0.6-mm substrate structure. Another single-layer or dual-layer substrate structure may then be bonded to the dual-layer structure made in accordance with the Mueller invention to make a DVD-14 or DVD-18 disc.
A problem with the Mueller method is that the substrate for the fully reflective layer has a diameter larger than that of the substrate that actually forms part of the finished DVD. A force is applied to the outer periphery of the larger substrate in order to push it away from the substrate structure to which the fully reflective layer has been transferred. Although the larger substrate can still be made on conventional CD-type molding equipment, the larger size does require some changes to be made in the molding equipment, as well as the handling mechanisms associated with the sputtering machine, etc.
Another problem with the Mueller method is that after the fully reflective layer is separated from the larger substrate, it is seen that some of the adhesive originally on the semireflective layer on the smaller substrate (the one that will become part of the finished DVD) has been squeezed out onto the edge of the deposited fully reflective layer. It is difficult to remove this adhesive without contaminating the substrate, and this causes problems in subsequent bonding.
It is an object of my invention to provide a method and apparatus for making dual-layer discs based on the Mueller method while at the same time solving the aforesaid problems.